Carbon-carbon brake discs are commonly used in aircraft brakes. The service life of a carbon-carbon brake is commonly quoted in terms of the number of landings the brake discs are expected to achieve before replacement, the number of landings being routinely logged for maintenance and airworthiness requirements. The expected number of landings is commonly used as a guarantee of minimum service life for heat packs that are sold or to calculate the cost per brake landing (CBL) where brake heat packs are leased to operators by the brake manufacturers.
Aircraft brakes, especially those employing carbon-carbon composite materials as the friction discs in a multi-disc arrangement, may suffer damage that can affect the performance of the brake during service. Routine inspection of brakes between scheduled services includes inspecting the wear pin to ensure sufficient material is available to enable the heat pack to safely absorb the energy of a stop without overheating and damage to the heat pack and surrounding components. Inspection of the wear pin will only reveal when the heat pack is reaching the end of its wear life and will not show other problems that can adversely affect performance of the brake such as broken discs, missing drive tenons, oxidation, uneven disc wear, brake drag or contamination.
It is desirable to have accurate information for determining the condition and predicting the life of carbon-carbon brake discs. This is important for safety as well as commercial reasons. For example, the high cost of carbon-carbon brake discs and delivery lead times in the order of six months makes provisioning of spares an important issue if inventory and hence working capital is to be minimized.
In use aircraft brakes are applied in five situations: when landing, when taxiing, when stationary, during pre-retract braking and, very rarely, during a rejected take off. This is an important point, for example, because it has been realized that the rate of wear of a carbon brake is dependent to a major extent on the number of brake operations effected and not on the energy dissipated during the operation. Thus, the wear during a taxi snub on a cold carbon-carbon brake is similar to that of a full landing.
The prior art relating to brake monitoring includes DE-A-3433236 which discloses a brake application monitoring device intended for use in vehicle or aircraft. This device comprises a chart recorder with traces driven by a transducer measuring the brake force, e.g. by sensing the hydraulic pressure applied, and an inertial sensor responsive to the actual deceleration of the vehicle or aircraft. However, there are several disadvantages with this proposal. For example, the brakes on an aircraft may require an applied pressure of approximately 150 psi simply to close the clearance in the brake before any braking effect is seen. However, a relatively small increase in that applied pressure may be all that is necessary to achieve the desired braking effect for a taxi snub. In many existing systems there is little or no sensing of brake pressure which means that modifications to those systems would be needed if brake pressure is to be used as a means of determining brake application. The use of an inertial sensor is not able to identify all kinds of braking operation carried out, for example check braking against engine thrust, and it might erroneously identify as a braking application a deceleration due to drag, wind effects or throttling back the engines as a braking application.
In the context of a braking system such as an aircraft wheel brake, it is known to measure physical parameters associated with the braking effect during an operation of the system with a view to regulating that effect.
For example, in a hydraulic servo operated aircraft brake system (autobrake) the extent to which the pilot has depressed the brake pedal, i.e. the brake demand, may be measured and the resulting hydraulic pressure applied to the brakes regulated to a value appropriate to the demand. In more complex examples, further parameters are measured. Thus, U.S. Pat. No. 4,790,606 to Reinecke discloses apparatus for measuring and/or regulating a braking force, which apparatus includes a deceleration sensor, a brake temperature sensor, a mass sensor and an evaluation means which uses the signals from these sensors to achieve the measurement and/or regulation. U.S. Pat. No. 4,592,213 to Rapoport discloses a braking system comprising temperature, friction and pressure sensors and means for comparing the signals from these sensors with predetermined values and automatically operating the braking system accordingly. U.S. Pat. No. 4,572,585 to Guichard and U.S. Pat. No. 4,043,607 to Signorelli et al also disclose systems of such a nature.
In some cases, the existence of excessively inefficient braking is signalled, e.g. by a warning signal to the pilot of an aircraft.